White particles for display, particle dispersion for display, display medium and display device

ABSTRACT

White particles for display including at least one of a chain or cyclic polysilane compound having a polysilane structure represented by the following Formula (I) or a halogen-substituted compound thereof: 
     
       
         
         
             
             
         
       
         
         
           
             wherein in Formula (I), A represents a phenyl group, B represents an alkyl group or a phenyl group, and n represents an integer of from 5 to 1000.

CROSS-REFERENCE TO RELATED APPLICATION

This application is based on and claims priority under 35 USC 119 fromJapanese Patent Application No. 2009-052028 filed Mar. 5, 2009.

BACKGROUND

The invention relates to white particles for display, a particledispersion for display, a display medium, and a display device.

RELATED ART

Conventionally, a display medium employing colored particles has beenknown as a re-writable display device. This display medium includes, forexample, a pair of substrates and particles that are enclosed betweenthe substrates such that the particles can move between the substratesin response to an electric field formed between the substrates.

SUMMARY

According to an aspect of the invention, there is provided whiteparticles for display including at least one of a chain or cyclicpolysilane compound having a polysilane structure represented by thefollowing Formula (I) or a halogen-substituted compound thereof:

wherein in Formula (I), A represents a phenyl group, B represents analkyl group or a phenyl group, and n represents an integer of from 5 to1000.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments of the present invention will be described indetail based on the following figures, wherein:

FIG. 1 is a schematic view of a display device according to a firstexemplary embodiment of the invention; and

FIGS. 2A and 2B are schematic views showing how the particles move uponapplication of a voltage between the substrate of the display deviceaccording to the first exemplary embodiment of the invention;

FIG. 3 is a schematic view of a display device according to a secondexemplary embodiment of the invention;

FIG. 4 is a diagram schematically showing the relationship between thevoltage and the degree of movement of particles (display density); and

FIG. 5 is a schematic view showing the relationship between the mode ofvoltage applied between the substrates of the display medium and themode of movement of particles.

DETAILED DESCRIPTION

(White Particles for Display and Particle Dispersion for Display)

The white particles for display according to the invention include atleast one of a linear or cyclic polysilane compound having a polysilanestructure represented by the following Formula (I) or ahalogen-substituted compound of the same.

Since the white particles for display according to the invention are achain or cyclic polysilane compound having the following structurerepresented by Formula (I) or a halogen-substituted compound of the same(hereinafter, these compounds may be referred to as a specificpolysilane compound) that exhibits a high refractive index (for example,1.65 or more) and a small specific gravity (for example, 1.1 or less),sedimentation of particles may be suppressed while improving thewhiteness. Further, when the white particles according to the inventionare applied to a display medium, the dispersed state of particles havinga high degree of whiteness may be easily maintained, thereby achievingdisplay of a white color with a high degree of whiteness in a stablemanner.

The white particles for display according to the invention may be formedfrom a powder of the specific polysilane compound itself, or may beresin particles with the specific polysilane compound dispersed orcompounded therein, or fixed (attached) thereon. When the whiteparticles for display according to the invention are resin particles,other components than the resin or the specific polysilane compound mayalso be included therein.

In the following, the components of the white particles for displayaccording to the invention will be described.

The specific polysilane compound is a chain or cyclic polysilanecompound having a polysilane structure represented by the followingFormula (I) or a halogen-substituted compound of the same.

In Formula (I), A represents a phenyl group, B represents an alkyl groupor a phenyl group, and n represents an integer of from 5 to 1,000.

The alkyl group represented by B may be an alkyl group having 1 to 22carbon atoms, preferably an alkyl group having 1 to 12 carbon atoms,more preferably an alkyl group having 1 to 6 carbon atoms. The alkylgroup may be a straight-chain alkyl group or a branched alkyl group.Examples of the alkyl group include a methyl group, an ethyl group, apropyl group, an n-butyl group, a sec-butyl group, a hexyl group, anoctyl group, an n-decyl group, an n-tetradecyl group, an n-hexadecylgroup, an n-octadecyl group, a stearyl group, an isopropyl group, anisobutyl group, and an isopentyl group.

The specific polysilane compound may be a chain polysilane compound or acyclic polysilane compound.

The chain polysilane compound preferably has a structure represented bythe following Formula (I-1A). In Formula (I-1A), n represents an integerof from 100 to 1,000, preferably from 200 to 800, more preferably from300 to 600. Examples of the terminal group of the chain polysilanecompound include a hydroxyl group, a halogen atom, a methyl group, anester group, an amino group, and a carboxyl group.

The cyclic polysilane compound preferably has a structure represented bythe following Formula (I-2A). This cyclic polysilane compound has fiveof the structure represented by Formula (I), but a compound having sixof the structure represented by Formula (I) is also preferred.

The specific polysilane compound may be a chain or cyclichalogen-substituted compound having a polysilane structure representedby Formula (I). The halogen-substituted compound refers to a polysilanecompound in which a phenyl group of at least one of the polysilanestructure represented by Formula (I) is substituted by a halogen atom(such as fluorine or chlorine). The halogen-substituted compoundexhibits an improved refractive index as compared with the unsubstitutedcompound. Specific examples of the halogen-substituted polysilanecompound include a chain polysilane compound represented by thefollowing Formula (I-1B) or a cyclic polysilane compound represented bythe following Formula (I-2B). In Formula (I-1B) and Formula (I-2B), Rrepresents a halogen atom, and n has the same definitions as that ofFormula (I-1A).

In the following, a method of synthesizing the specific polysilanecompound will be described, taking the compound having a polysilanestructure represented by Formula (I) as an example. The polysilanecompound having the structure represented by Formula (I) may besynthesized according to a known method without being particularlylimited. For example, the specific polysilane compound may be obtainedby polymerizing a monomer that is obtained by reacting a siliconcompound having a side chain A of the above polysilane with a Grignardreagent, which is a metal compound having a side chain B.

In order to regulate the molecular weight distribution (Mw/Mn) of thepolysilane compound obtained by polymerizing the monomer as describedabove to 1.10 or less, it is preferred to recover the obtainedpolysilane compound by a precipitation-fractionation method. Here, thespecific polysilane is a nonpolar polymer having no polar group in themolecule thereof. Therefore, the specific polysilane compound can beobtained by a precipitation-fractionation method, in which a poorsolvent is gradually added to a solution of a good solvent in which thepolysilane compound is dissolved, and then recovering a precipitationupon formation thereof.

The good solvent and the poor solvent used in the above process are notparticularly limited, and may be appropriately selected depending on thestructure of the polysilane compound. Examples of the good solventinclude nonpolar solvents such as toluene, isooctane, and n-decane.Examples of the poor solvent include polar solvents such as isopropylalcohol, ethanol, methanol, and water. Two or more kinds of poor solventmay be added to the good solvent including the polysilane compound.Namely, the weight average molecular weight of the polysilane compoundobtained as a precipitate in the solvent differs depending on thepolarity of the poor solvent. Therefore, the poor solvent is preferablyselected so that a precipitation of polysilane compound having a desiredweight average molecular weight can be obtained. In thisprecipitation-fractionation method, a large-scale production system isnot necessary and a polysilane compound can be produced in large amountsin a simple and inexpensive process.

Further, by classifying the polysilane compound obtained in the aboveprocess for several times, different types of polysilane compound havingdifferent weight average molecular weights are obtained. Among these,several types of polysilane compound having different weight averagemolecular weights are selected and dissolved in a common solvent. Thecommon solvent is not particularly limited as long as it is a goodsolvent, and may be appropriately selected according to the structure ofthe polysilane compound.

Other representative methods of synthesizing the specific polysilanecompound include a method known as a Kipping method as described inJapanese Patent Application Laid-Open (JP-A) No. 9-324053, in whichdialkyl dihalosilane or dihalotetraalkyl disilane in a toluene solventis subjected to reductive coupling using an alkali metal such as metalsodium, while vigorously stirring the solvent at a temperature of 100°C. or more.

Other applicable methods include a method of anion-polymerizing adisilene masked with biphenyl or the like (Japanese Patent ApplicationLaid-Open (JP-A) No. 1-230638); a method of performing ring-openingpolymerization of a cyclic silane (JP-A No. 5-170913); a method ofperforming dehydrogenative condensation polymerization of a hydrosilaneusing a transition metal complex catalyst (JP-A No. 7-17753); a methodof producing a polysilane by performing electrode reduction of adihalosilane at room temperature or lower (JP-A No. 7-309953); a methodof performing dehalogenation condensation polymerization of a halosilaneusing magnesium as a reduction agent (known as magnesium reductionmethod, see WO98/29476, JP-A No. 2003-277507 and JP-A No. 2005-36139).

In particular, the magnesium reduction method has advantages in that (1)a polysilane can be synthesized from an inexpensive raw material in astable manner using a general-purpose chemical synthesis system, therebyenabling the synthesis in a safe and cost-effective manner; (2)inclusion of impurities such as sodium or substances that is insolubleto an organic solvent, which are not desirable for applications such asoptical/electronic materials, does not occur; (3) a polysilane having aless variable molecular weight, a high degree of solubility with respectto an organic solvent and a high degree of transparency can be obtained;and (4) a polysilane can be obtained at high yield. Accordingly, thepolysilane compound may be obtained by the magnesium reduction method.In this method, a polysilane compound is synthesized by polymerizing ahalosilane under the presence of at least a magnesium metal component.

When the specific polysilane compound is dispersed or compounded inresin particles or fixed (attached) on the resin particles, the amountthereof is preferably from 3 to 99% by weight, more preferably from 10to 70% by weight, with respect to the amount of resin that forms theresin particles.

The resin that forms the resin particles may be a thermoplastic resin ora thermosetting resin.

Examples of the thermoplastic resin include homopolymers or copolymersof styrenes (such as styrene and chlorostyrene), monoolefins (such asethylene, propylene, butylene and isoprene), vinyl esters (such as vinylacetate, vinyl propionate, vinyl benzoate and vinyl butyrate),α-methylene aliphatic monocarboxylates (such as methyl acrylate, ethylacrylate, butyl acrylate, dodecyl acrylate, octyl acrylate, phenylacrylate, methyl methacrylate, ethyl methacrylate, butyl methacrylateand dodecyl methacrylate), vinyl ethers (such as vinyl methyl ether,vinyl ethyl ether and vinyl butyl ether), and vinyl ketones (such asvinyl methyl ketone, vinyl hexyl ketone and vinyl isopropenyl ketone).

Examples of the thermosetting resins include crosslinked resins (such asa crosslinked copolymer including divinyl benzene as a main componentand a crosslinked polymethyl methacrylate), phenol resins, urea resins,melamine resins, polyester resins and silicone resins.

The resin that forms the resin particles may be a polymer (resin) havinga charging group. The polymer having a charging group refers to apolymer having a cationic group or an anionic group as a charging group.Examples of the cationic group include an amino group and a quaternaryammonium group (and a salt of these groups). These cationic groupspositively charge the particles.

Examples of the anionic group include a phenol group, a carboxyl group,a carboxylate group, a sulfonic group, a sulfonate group, a phosphoricgroup, a phosphate group, and a tetraphenyl boron group (and a salt ofthese groups). These anionic groups negatively charge the particles.

The polymer having a charging group may be a homopolymer of a monomerhaving a charging group, or a copolymer of a monomer having a charginggroup and other monomer (a monomer having no charging group).

The monomer having a charging group may be a monomer having a cationicgroup (hereinafter, referred to as a cationic monomer) or a monomerhaving an anionic group (hereinafter, referred to as an anionicmonomer). In the following, the description “(meth)acrylate” or the likerefers to both acrylate and methacrylate.

Examples of the cationic monomer include (meth)acrylates having analiphatic amino group, such as N,N-dimethylaminoethyl(meth)acrylate,N,N-diethylaminoethyl (meth)acrylate,N,N-dibutylaminoethyl(meth)acrylate, N,N-hydroxyethylaminoethyl(meth)acrylate, N-ethylaminoethyl(meth)acrylate,N-octyl-N-ethylaminoethyl (meth)acrylate, andN,N-dihexylaminoethyl(meth)acrylate; aromatic-substituted ethylenemonomers having a nitrogen-containing group, such asdimethylaminostyrene, diethylaminostyrene, dimethylaminomethylstyreneand dioctylaminostyrene; nitrogen-containing vinyl ether monomers, suchas vinyl-N-ethyl-N-phenylaminoethyl ether,vinyl-N-butyl-N-phenylaminoethyl ether, triethanolamine divinyl ether,vinyl diphenyl aminoethyl ether, N-vinyl hydroxyethyl benzamide, andm-aminophenyl vinyl ether; vinylamine; pyrroles such as N-vinyl pyrrole;pyrrolines such as N-vinyl-2-pyrroline and N-vinyl-3-pyrroline;pyrrolidines such as N-vinyl pyrrolidine, vinylpyrrolidine amino ether,and N-vinyl-2-pyrrolidone; imidazoles such as N-vinyl-2-methylimidazole; imidazolines such as N-vinyl imidazoline, indoles such asN-vinyl indole, indolines such as N-vinyl indoline, carbazoles such asN-vinyl carbazole and 3,6-dibromo-N-vinyl carbazole, pyridines such as2-vinyl pyridine, 4-vinyl pyridine and 2-methyl-5-vinyl pyridine,piperidines such as (meth)acrylic piperidine, N-vinyl piperidone andN-vinyl piperadine, quinolines such as 2-vinyl quinoline and 4-vinylquinoline, pyrazoles such as N-vinyl pyrazole and N-vinyl pyrazoline,oxazoles such as 2-vinyl oxazole, and oxazines such as 4-vinyl oxazineand morpholinoethyl(meth)acrylate.

In view of versatility, the cationic monomer is preferably a(meth)acrylate having an aliphatic amino group such asN,N-dimethylaminoethyl(meth)acrylate andN,N-diethylaminoethyl(meth)acrylate. In particular, these monomers arepreferably used in the form of a quaternary ammonium salt, before orafter the polymerization. The quaternary ammonium salt may be obtainedby allowing the monomer to react with an alkyl halide or a tosylate.

Examples of the anionic monomer include carboxylic acid monomers such as(meth)acrylic acid, crotonic acid, itaconic acid, maleic acid, fumaricacid, citraconic acid, anhydrides and monoalkyl esters of thesemonomers, and vinyl ethers having a carboxyl group such as carboxylethylvinyl ether and carboxylpropyl vinyl ether;

sulfonic acid monomers such as styrene sulfonic acid,2-acrylamide-2-methylpropane sulfonic acid, 3-sulfopropyl(meth)acrylicacid ester, bis-(3-sulfopropyl)-itaconic acid ester, a salt of thesemonomers, as well as other sulfonic acid monoesters such as2-hydroxyethyl(meth)acrylic acid or a salt of these monomers; and

phosphoric acid monomers such as vinyl phosphoric acid, vinyl phosphate,acid phosphoxyethyl(meth)acrylate, acid phosphoxypropyl(meth)acrylate,bis(methacryloyoxyethyl)phosphate, diphenyl-2-methacyloyloxyethylphosphate, diphenyl-2-acryloyloxyethyl phosphate,dibutyl-2-methacryloyloxyethyl phosphate, dibutyl-2-acryloyloxyethylphosphate, and dioctyl-2-(meth)acryloyloxyethyl phosphate.

The anionic monomer is preferably a monomer having (meth)acrylic acid orsulfonic acid, which is more preferably in the form of an ammonium saltbefore or after the polymerization. The ammonium salt may be obtained byallowing the monomer to react with a tertiary amine or a quaternaryammonium hydroxide.

Examples of the monomer having no charging group include a nonionicmonomer such as (meth)acrylonitrile, alkyl(meth)acrylate,(meth)acrylamide, ethylene, propylene, butadiene, isoprene, isobutylene,N-dialkyl substituted (meth)acrylamide, styrene, styrene derivatives,vinyl carbazole, polyethylene glycol mono(meth)acrylate, vinyl chloride,vinylidene chloride, isoprene, butadiene, N-vinyl pyrrolidone,hydroxyethyl(meth)acrylate, and hydroxybutyl(meth)acrylate.

The copolymerization ratio of the monomer having a charging group andthe monomer having no charging group may be determined depending on thedesired charge amount of the particles, and is typically selected fromthe range of 1:100 to 100:1 (molar ratio, the monomer having a charginggroup : the monomer having no charging group).

The weight average molecular weight of the resin that forms the resinparticles is preferably from 1,000 to 1,000,000, more preferably from10,000 to 200,000.

Other materials that may be compounded in the resin particles include acharge control agent or a magnetic material. Examples of the chargecontrol agent include known compounds used for electrophotographic tonermaterials, such as cetylpyridinium chloride, quaternary ammonium saltssuch as BONTRON P-51, BONTRON P-53, BONTRON E-84 and BONTRON E-81 (tradename, manufactured by Orient Chemical Industries, Co., Ltd.), salicylicmetal complexes, phenol condensates, tetraphenyl compounds, metal oxideparticles, or metal oxide particles having the surface treated with acoupling agent.

In the invention, a silicone polymer may be bound to (or applied on) thesurface of the powder of the specific polysilane compound or resinparticles having the aforementioned structure (hereinafter, referred toas white mother particles). The silicone polymer refers to a polymerincluding a silicone chain, preferably a polymer having a silicone chain(silicone graft chain) as a side chain, with respect to the main chainof the polymer.

One preferable example of the silicone polymer is a copolymer obtainedfrom a silicone chain component and optionally at least one selectedfrom an optional reactive component, a component having a charginggroup, and a component having no charging group. The raw material forthese components (in particular, a silicone chain component) may be amonomer or a macromonomer. The macromonomer collectively refers to anoligomer (with a polymerization degree of 2 to about 300) or a polymerhaving a polymerizable functional group, and exhibits thecharacteristics of both of a polymer and a monomer.

Examples of the silicone chain component include a dimethyl siliconemonomer having a (meth)acrylate group at one terminal end thereof, suchas SILAPLANE FM-0711, FM-0721 and FM-0725 (trade name, manufactured byChisso Corporation), X-22-174DX, X-22-2426 and X-22-2475 (trade name,manufactured by Shin-Etsu Chemical Co., Ltd.).

Examples of the reactive component include a glycidyl(meth)acrylatehaving an epoxy group, or an isocyanate monomer having an isocyanategroup (such as KARENZ AOI and KARENZ MOI, trade name, Showa Denko K.K.)

Examples of the copolymerization component having a charging group orthe copolymerization component having no charging group include themonomers having a charging group or the monomers having no charginggroup, such as those as mentioned above concerning the polymer having acharging group.

The silicone polymer may include a silicone chain component in an amountof from 3 to 60% by weight, preferably from 5 to 40% by weight withrespect to the total amount of the polymer. When the amount of thesilicone chain component is within the above range, stabledispersibility of the particles can be obtained while achieving otherproperties (such as imparting the charge polarity or controlling thecharge amount).

Another example of the silicone polymer is a silicone compound having anepoxy group at one terminal end thereof (represented by the followingFormula 1). Examples of the silicone compound having an epoxy group atone terminal end include X-22-173DX (trade name, manufactured byShin-Etsu Chemical Con, Ltd.)

In Formula 1, R₁′ represents a hydrogen atom or an alkyl group havingcarbon atoms of 1 to 4, n represents a natural number of 1 to 1,000 forexample, preferably 3 to 100, and x represents an integer of 1 to 3.

Yet another preferable example of the silicone polymer is a copolymerobtained from at least a dimethyl silicone monomer having a(meth)acrylate group at one terminal end thereof as represented by thefollowing Formula 2, such as SILAPLANE FM-0711, FM-0721 and FM-0725(trade name, manufactured by Chisso Corporation) and X-22-174DX,X-22-2426 and X-22-2475 (trade name, manufactured by Shin-Etsu ChemicalCo., Ltd.), and a glycidyl(meth)acrylate or an isocyanate monomer, suchas KARENZ AOI and KARENZ MOI (trade name, Showa Denko K.K.)

In Formula 2, R₁ represents a hydrogen atom or a methyl group, R₁′represents a hydrogen atom or an alkyl group having carbon atoms of 1 to4, n represents a natural number of 1 to 1,000 for example, preferably 3to 100, and x represents an integer of 1 to 3.

The weight average molecular weight of the silicone polymer ispreferably from 500 to 1,000,000, more preferably from 1,000 to1,000,000.

The silicone polymer may be bonded to (or applied on) the white motherparticles by a coacervation method.

The coacervation method includes dispersing the white mother particlesas prepared by a known process (such as pulverization, coacervation,dispersion-polymerization or suspension-polymerization) in a firstsolvent in which the silicone polymer is dissolved, dropping a secondsolvent to the first solvent to emulsify the same, and then removing thefirst solvent and allowing the silicone polymer to precipitate on thesurface of white mother particles and to react so as to be bonded to orapplied on the surface of the white mother particles.

Examples of the first solvent include isopropyl alcohol (IPA), methanol,ethanol, butanol, tetrahydrofuran, ethyl acetate, and butyl acetate.Among these, isopropyl alcohol (IPA) is preferable since it can impartthe particles stable dispersibility and charging properties. The secondsolvent is preferably a silicone oil.

The method of bonding or applying the silicone polymer to the whitemother particles is not particularly limited to the above process.

The white particles for display according to the invention may have avolume average particle size of from 0.1 to 10 μm, preferably from 0.2to 5 μm, more preferably from 0.3 to 1 μm.

When the volume average particle size to be measured is 2 μm or more,the measurement is conducted with a COULTER COUNTER TA-II (trade name,manufactured by Beckman Coulter, Inc.) using ISOTON-II (trade name,manufactured by Beckman Coulter, Inc.) as an electrolyte.

The measurement can be conducted by a method including adding 0.5 to 50mg of a sample to 2 ml of an aqueous solution including a surfactant asa dispersant, preferably 0.5% of sodium alkylbenzene sulfonate, andadding the same to 100 to 150 ml of the aforementioned electrolyte;subjecting this electrolyte in which the sample is suspended to adispersion treatment for 1 minute using an ultrasonic disperser; andthen measuring the particle size distribution of the particles having aparticle size of 2.0 to 60 μm using the COULTER COUNTER TA-II with anaperture having a diameter of 100 μm. The number of particles formeasurement is 50,000.

Based on the particle size distribution as measured above, anaccumulation distribution is delineated from the smaller size side, ineach of volume and number with respect to the divided particle sizerange (channel). The particle size at which the volume accumulation is16% is determined as D16v, and the particle size at which theaccumulated number is 16% is determined as D16p. Similarly, the particlesize at which the volume accumulation is 50% is determined as D50v, andthe particle size at which the accumulated number is 50% is determinedas D50p. Further, the particle size at which the volume accumulation is84% is determined as D84v, and the particle size at which theaccumulated number is 84% is determined as D84p. The volume averageparticle size is D50v.

Using the above indicators, the volume average particle sizedistribution index (GSDv) is calculated by (D84v/D16v)^(1/2); the numberaverage particle size distribution index (GSDp) is calculated by(D84p/D16p)^(1/2); and the number average particle size distributionindex at the smaller size side (lower GSDp) is calculated by{(D50p)/(D16p)}.

When the volume average particle size to be measured is less than 2 μm,the measurement is conducted with a laser scattering particle sizemeasurement device (trade name: LA-700, manufactured by Horiba, Ltd.)Specifically, a sample in the form of a dispersion with a solid contentof 2 g is prepared and ion exchange water is added to the sample to givethe total amount of 40 ml, and the sample is placed in a cell to give anappropriate concentration. After 2 minutes, when the concentration inthe cell is stabilized, the measurement is conducted. The volume averageparticle size as measured at each channel is accumulated from the sideof smaller particles size, and the particle size at which theaccumulation is 50% is determined as the volume average particle size.

The amount of a powder such as an external agent is measured by adding 2g of a sample in 50 ml of 0.5% aqueous solution of a surfactant,preferably sodium alkylbenzene sulfonate, dispersing the same using anultrasonic disperser (1,000 Hz) for 2 minutes, and then conducting themeasurement in accordance with the method as mentioned above.

The white particles for display according to the invention may be usedas mobile particles that move in response to an electric field, or asnon-mobile particles that do not move in response to an electric field.When the white particles for display according to the invention are usedas mobile particles that move in response to an electric field, theparticles are formed by using a polymer having a charging group as theresin that forms the particles, or by using a component having acharging group for the silicone polymer. On the other hand, when thewhite particles for display according to the invention are used asnon-mobile particles that do not move in response to an electric field,the particles are formed by using a polymer having no charging group asthe resin that forms the particles, or by using a component having nocharging group for the silicone polymer. The charging group refers to agroup having a tendency of being ionized by acid or base dissociation,and examples thereof include an amino group and a carboxyl group.

The particle dispersion for display that employs the white particlesaccording to the invention includes the white particles for displayaccording to the invention, and a dispersing medium in which theparticles are dispersed. The particle dispersion for display accordingto the invention may include other particles for display (colorparticles). As necessary, the particle dispersion may further include anacid, an alkali, a salt, a dispersant, a dispersion stabilizer, astabilizer for anti-oxidization or UV absorption, an antibacterialagent, an antiseptic agent, or the like.

The dispersing medium may be any material that can be used for a displaymedium, but when the aforementioned silicone polymer is bonded to orcoated on the surface of the white particles, a silicone oil ispreferably used.

Examples of the charge control agent include an ionic or nonionicsurfactant, a block or graft copolymer having a lipophilic portion and ahydrophilic portion, a compound having a polymeric skeleton of a cyclic,stellate, or dendritic structure, a salicyclic metal complex, a catecholmetal complex, a metal-containing bisazo dye, a tetraphenyl boratederivative, and a copolymer of a polymerizable macromer (such asSILAPLANE, trade name, manufactured by Chisso Corporation) and ananionic monomer or a cationic polymer.

Examples of the nonionic surfactant include polyoxyethylene nonyl phenylether, polyoxyethylene octyl phenyl ether, polyoxyethylene dodecylphenyl ether, polyoxyethylene alkyl ether, polyoxyethylene fatty acidester, sorbitan fatty acid ester, polyoxyethylene sorbitan fatty acidester, and fatty acid alkylol amide.

Examples of the anionic surfactant include alkyl benzene sulfonate,alkyl phenyl sulfonate, alkyl naphthalene sulfonate, higher fatty acidsalt, a sulfate of higher fatty acid ester, and a sulfonate of higherfatty acid ester.

Examples of the cationic surfactant include a primary to tertiary aminesalt, or a quaternary ammonium salt.

The charge control agents is preferably included in an amount of from0.01 to 20% by weight, particularly preferably from 0.05 to 10% byweight, with respect to the solid content of the particles.

The particles for display and the particle dispersion for displayaccording to the invention are applicable to an electrophoresis displaymedium, a liquid toner for use in an electrophotographic systememploying a liquid developing system, or the like.

(Display Medium and Display Device)

In the following, exemplary embodiments of the display medium and thedisplay device will be described.

First Exemplary Embodiment

FIG. 1 is a schematic view of a display device according to the firstexemplary embodiment. FIGS. 2A and 2B are schematic views showing howthe particles move upon application of a voltage between the substratesof the display device according to the first exemplary embodiment of theinvention.

Display device 10 according to the first exemplary embodiment employscolor particles having a color other than white as mobile particles 34,and the white particles for display according to the invention asreflective particles 36. It is also possible to employ the whiteparticles for display according to the invention as mobile particles 34.

Display device 10 includes, as shown in FIG. 1, a display medium 12, avoltage application unit 16 that applies a voltage to display medium 12,and a control unit 18.

Display medium 12 includes a display substrate 20 that displays animage; a rear substrate 22 that is positioned opposite to displaysubstrate 20 with a space; spacers 24 that maintain the substrates to bepositioned with a specified space and divide the space between thesubstrates into plural cells; mobile particles 34 included in each cell;and reflective particles 36 having a different optical reflectionproperty than that of mobile particles 34.

The cell as mentioned above refers to a space surrounded by displaysubstrate 20, rear substrate 22, and spacers 24. A dispersing medium 50is included in the cell. Mobile particles 34 consisting of plural kindsof particles are dispersed in dispersing medium 50, and move betweendisplay substrate 20 and rear substrate 22 through the gaps amongreflective particles 36 in response to an electric field formed in thecell.

In this exemplary embodiment, mobile particles 34 included in each cellare described as having a single specific color and have been previouslytreated to be either positively or negatively charged.

It is also possible to configure display medium 12 so that display canbe performed in each pixel, by providing spacers 24 to form a cell so asto correspond to each pixel of an image to be displayed.

For the purpose of simplification, this exemplary embodiment will bedescribed referring to a drawing that shows only a single cell. In thefollowing, details of each component will be described.

Display substrate 20 includes, on a support 38, a front electrode 40 anda surface layer 42 in this order. Rear substrate 22 includes, on asupport 44, a rear electrode 46 and a surface layer 48 in this order.

Only display substrate 20, or both display substrate 20 and rearsubstrate 22 are transparent. In this exemplary embodiment, beingtransparent refers to having a transmittance with respect to visiblerays of 60% or more.

Materials for support 38 and support 44 include glass and plastics suchas polyethylene terephthalate resin, polycarbonate resin, acrylic resin,polyimide resin, polyester resin, epoxy resin, and polyether sulfoneresin.

Materials for front electrode 40 and rear electrode 46 includes oxidesof indium, tin, cadmium, antimony or the like, composite oxides such asITO, metals such as gold, silver, copper or nickel, and organicmaterials such as polypyrrole or polythiophene. Front electrode 40 andrear electrode 46 may be formed from a material such as those to asingle film, a mixed film or a composite film, by a method ofevaporation, sputtering, coating or the like. The thickness of frontelectrode 40 and rear electrode 46 is typically from 100 to 2,000angstroms, when these electrodes are formed by evaporation orsputtering. Front electrode 40 and rear electrode 46 may be formed in adesired patterned manner by a known method such as etching that isperformed to form coventional liquid crystal displays or printed boards.For example, front electrode 40 and rear electrode 46 may be formed in amatrix pattern or a striped pattern that enables passive matrix driving.

Front electrode 40 may be embedded in support 38, or rear electrode 46may be embedded in support 44. In this case, the material for supports38 and 44 is selected in accordance with the composition of each kind ofmobile particles 34.

Front electrode 40 and rear electrode 46 may be separated from displaysubstrate 20 and rear substrate 22, and positioned outside displaymedium 12.

In the above description, both display substrate 20 and rear substrate22 are provided with an electrode (front electrode 40 and rear electrode46). However, it is also possible to provide an electrode only to onesubstrate for performing active matrix driving.

In order to enable active matrix driving, a thin film transistor (TFT)may be provided to support 38 and support 44 at each pixel. The TFT ispreferably formed on rear substrate 22 rather than on display substrate20, since formation of a multilayer wiring or packaging may be readilyconducted.

When display medium 12 is driven by the passive matrix system,configuration of display device 10 including display medium 12 can besimplified. When display medium 12 is driven by the active matrixsystem, the display speed can be increased as compared with the passivematrix system.

When front electrode 40 and rear electrode 46 are formed on support 38and support 44, respectively, dielectric films as surface layers 42 and48 are optionally formed on front electrode 40 and rear electrode 46,respectively, in order to prevent breakage of the electrodes or leakagebetween the electrodes that causes fixation of mobile particles 34.

Materials for surface layers 42 and 48 include polycarbonate, polyester,polystyrene, polyimide, epoxy, polyisocyanate, polyamide, polyvinylalcohol, polybutadiene, polymethylmethacrylate, copolymerized nylon,UV-cured acrylic resin, and fluorocarbon resin.

Other than the aforementioned insulating materials, an insulatingmaterial in which a charge transporting substance is included may alsobe used. Inclusion of a charge transporting substance may provide sucheffects as improving the charging properties of the particles by chargeinjection, allowing the charges to leak from the particles when theamount of the charges is exceedingly increased, so as to stabilize theamount of charges to the particles.

Examples of the charge transporting substance include hole transportingsubstances such as hydrazone compounds, stilbene compounds, pyrazolinecompounds, and arylamine compounds; and electron transporting substancessuch as fluorenone compounds, diphenoquinone compounds, pyranecompounds, and zinc oxide.

A self-supporting resin having a charge transporting property may alsobe used. Specific examples thereof include polyvinyl carbazole, and apolycarbonate obtained by polymerizing a specific hydroxyarylamine andbischloroformate, as described in the U.S. Pat. No. 4,806,443.

Since the dielectric film may affect the charging properties or fluidityof the particles, the material thereof is selected in accordance withthe composition of the particles, or the like. Since display substrate20 needs to be transparent, the surface layer of display substrate 20 ispreferably formed from a transparent material.

Spacers 24 that maintain a space between display substrate 20 and rearsubstrate 22 are formed so as not to impair the transparency of displaysubstrate 20, and are formed from thermoplastic resin, thermosettingresin, electron beam-curing resin, photo-curing resin, rubber, metal, orthe like.

Spacers 24 may be formed in an integrated manner with either displaysubstrate 20 or rear substrate 22. In this case, spacers 24 may beformed by subjecting support 38 or support 44 to an etching treatment,laser treatment, pressing treatment using a predetermined pattern, orprinting treatment.

In this case, spacers 24 are formed on either side of display substrate20 or rear substrate 22, or on both sides.

Spacers 24 may have a color or colorless, but is preferably colorlessand transparent so as not to affect the image displayed on displaymedium 12. In this case, for example, spacers 24 are formed from atransparent polystyrene resin, polyester resin, or acrylic resin.

When spacers 24 are in a particulate form, spacers 24 may also be formedfrom glass particles, as well as particles of a transparent polystyreneresin, polyester resin, or acrylic resin.

Being transparent here refers to having a transmittance of 60% or morewith respect to visible rays.

Mobile particles 34 included in display medium 12 may be dispersed in apolymeric resin as dispersing medium 50. This polymeric resin may be apolymeric gel or a polymeric polymer.

Examples of the polymeric gel include most types of synthesis polymericgel, and polymeric gels derived from a natural polymer such as agarose,agaropectin, amylose, sodium alginate, propylene glycol alginate,isolichenan, insulin, ethyl cellulose, ethylhydroxy ethyl cellulose,curdlan, casein, carrageenan, carboxymethyl cellulose, carboxymethylstarch, callose, agar, chitin, chitosan, silk fibroin, guar gum, quinceseed, crown-gall polysaccharide, glycogen, glucomannan, keratan sulfate,keratin protein, collagen, cellulose acetate, gellan gum, schizophyllan,gelatin, ivory palm mannan, tunicin, dextran, dermatan sulfate, starch,tragacanth gum, nigeran, hyaluronic acid, hydroxyethyl cellulose,hydroxypropyl cellulose, pusturan, funoran, decomposed xyloglucan,pectin, porphyran, methyl cellulose, methyl starch, laminaran, lichenan,lentinan, and locust beam gum.

Further examples include polymers including a functional group ofalcohol, ketone, ether, ester or amide in the repeating unit, such aspolyvinyl alcohol, poly(meth)acrylamide, derivatives thereof, polyvinylpyrrolidone, polyethylene oxide, or copolymers including these polymers.

Among these, gelatin, polyvinyl alcohol and poly(meth)acrylamide arepreferably used in view of production stability and electrophoreticproperties.

The aforementioned polymeric resin is preferably used as dispersingmedium 50 together with the aforementioned insulating material.

Mobile particles 34 included in each cell and consisting of plural kindsof particles are dispersed in dispersing medium 50, and move betweendisplay substrate 20 and rear substrate 22 in response to the strengthof an electric field formed in the cell.

Particles that constitute mobile particles 34 may be particles of aninsulating metal oxide such as glass, alumina or titanium oxide,particles of thermoplastic resin or thermosetting resin, resin particleswith a colorant fixed on the surface thereof particles of thermoplasticresin or thermosetting resin including an insulating colorant therein,particles of metal colloid having a plasmon coloring function, or thelike.

Examples of the thermoplastic resin for the mobile particles includehomopolymers or copolymers of styrenes (such as styrene andchlorostyrene), mono-olefins (such as ethylene, propylene, butylene andisoprene), vinyl esters (such as vinyl acetate, vinyl propionate, vinylbenzoate and vinyl butyrate), α-methylene aliphatic monocarboxylates(such as methyl acrylate, ethyl acrylate, butyl acrylate, dodecylacrylate, octyl acrylate, phenyl acrylate, methyl methacrylate, ethylmethacrylate, butyl methacrylate and dodecyl methacrylate), vinyl ethers(such as vinyl methyl ether, vinyl ethyl ether and vinyl butyl ether),and vinyl ketones (such as vinyl methyl ketone, vinyl hexyl ketone andvinyl isopropenyl ketone).

Examples of the thermosetting resins for the mobile particles includecrosslinked resins (such as a crosslinked copolymer including divinylbenzene as a main component and a crosslinked polymethyl methacrylate),phenol resins, urea resins, melamine resins, polyester resins andsilicone resins. Particularly representative binder resins includepolystyrene, styrene-alkyl acrylate copolymer, styrene-alkylmethacrylate copolymer, styrene-acrylonitrile copolymer,styrene-butadiene copolymer, styrene-maleic anhydride copolymer,polyethylene, polypropylene, polyester, polyurethane, epoxy resin,silicone resin, polyamide, modified rosin, and paraffin wax.

Examples of the colorant include organic or inorganic pigments oroil-soluble dye. Examples of known colorants include magnetic powder ofmagnetite, ferrite or the like, carbon black, titanium oxide, magnesiumoxide, zinc oxide, phthalocyanine copper cyano colorant, azo yellowcolorant, azo magenta colorant, quinacridone magenta colorant, redcolorant, green colorant and blue colorant. Specific example thereofinclude aniline blue, calco oil blue, chrome yellow, ultramarine blue,DuPont oil red, quinoline yellow, methylene blue chloride,phthalocyanine blue, malachite green oxalate, lamp black, rose bengal,C.I. pigment red 48:1, C.I. pigment red 122, C.I. pigment red 57:1, C.I.pigment yellow 97, C.I. pigment blue 15:1, C.I. pigment blue 15:3. Thesecolorants may be used alone or in combination.

As necessary, a charge control agent may be mixed in the resin for themobile particles. Known charge control agents for use ineletrophotographic toner materials are applicable, and examples thereofinclude cetylpyridinium chloride, quaternary ammonium salts such asBONTRON P-51, BONTRON P-53, BONTRON E-84 and BONTRON E-81 (trade name,manufactured by Orient Chemical Industries, Co., Ltd.), salicylic metalcomplexes, phenol condensates, tetraphenyl compounds, metal oxideparticles, and metal oxide particles having the surface treated with acoupling agent of various kinds.

As necessary, a magnetic material may be mixed in the mobile particles,or applied on the surface thereof. The magnetic material may be anorganic or inorganic magnetic material that may have an optional coatingof a colorant. A transparent magnetic material, especially a transparentorganic magnetic material is preferred since it does not inhibitcoloring of the colored pigment, and has a specific gravity that is lessthan that of the organic magnetic material.

A colored magnetic powder, such as the small colored magnetic powder asdisclosed in JP-A No. 2003-131420, may be used as the magnetic material.For example, a magnetic powder including a core magnetic particle and acolor layer formed on the core magnetic particles may be used. In thiscase, the color layer may be selected so as to color the magnetic powderwith a pigment or the like in an opaque manner, but a thin film thatexhibits a color by light interference is preferred. This thin film isformed from a colorless material such as SiO₂ or TiO₂ to a thicknessequivalent to a wavelength of light, and reflects light in a selectivemanner due to light interference inside the thin film.

As necessary, an external additive may be attached to the surface of themobile particles. The color of the external additive is preferablytransparent so as not to affect the color of the mobile particles.

Materials for the external additive include particles of a metal oxidesuch as silicon oxide (silica), titanium oxide or alumina. The mobileparticles may be surface-treated with a coupling agent or silicone oil,in order to adjust the charging property, fluidity or environmentdependency of the mobile particles.

Examples of the coupling agent include positively charged ones such asaminosilane coupling agents, aminotitanium coupling agents and nitrilecoupling agents, and negatively charged ones that do not include anitrogen atom (consisting of atoms other than a nitrogen atom) such assilane coupling agent, titanium coupling agent, epoxysilane couplingagent, and acrylsilane coupling agent.

Examples of the silicone oil include positively charged ones such asamino-modified silicone oil, and negatively charged ones such asdimethyl silicone oil, alyl-modified silicone oil,α-methylsulfone-modified silicone oil, methylphenyl silicone oil,chlorophenyl silicone oil, and fluorine-modified silicone oil.

These coupling agents or silicone oils may be selected depending on thedesired resistivity of the external additive.

Among the above external additives, hydrophobic silica and hydrophobictitanium oxide that are well known in the art are preferred, and atitanium compound obtained by allowing TiO(OH)₂ to react with a silanecompound such as a silane coupling agent, as described in JP-A No.10-3177, is particularly preferred. Any of chlorosilanes, alkoxysilanes, silazanes, or speciality silylation reagents may be used as thesilane compound. This titanium compound may be produced by allowingTiO(OH)₂ produced in a wet process to react with a silane compound or asilicone oil, and then drying the reactant. Since this process does notinclude sintering at a temperature of as high as several hundreds, nostrong bond is formed among the Ti atoms and no aggregation occurs.Therefore, the mobile particles are in the form of primary particles.Further, since TiO(OH)₂ is directly allowed to react with a silanecompound or silicone oil, it is possible to control the chargingproperties by adjusting the amount of silane compound or silicone oilused for the treatment and even more improved charging properties can beachieved as compared with those of conventional titanium oxide.

The volume average particle size of the external additive is notparticularly limited, but is typically from 5 nm to 100 nm, morepreferably from 10 nm to 50 nm.

The compounding ratio of the external additive and the mobile particlesmay be determined depending on the size of the mobile particles and theexternal additive. When the amount of the external additive is toolarge, part of the external additive may be detached from the surface ofmobile particles and attach to the surface of other mobile particles,thereby failing to obtain desired charging properties. Typically, theamount of the external additive may be from 0.01 to 3 parts by weight,preferably from 0.05 to 1 part by weight, with respect to 100 parts byweight of the mobile particles.

The external additive may be added to only one kind of the mobileparticles, or may be added to two or more kinds, or all kinds of themobile particles. The addition of the external additive to the surfaceof the mobile particles is preferably conducted by striking the externaladditive in the surface of the mobile particles with impact strength, orheating the surface of the mobile particles, so that the externaladditive is tightly fixed on the surface of the mobile particles. Inthis way, it is possible to inhibit external additive from beingdetached from the mobile particles and forming an aggregate of theexternal additive having different polarities that is hard to bedissociated by an electric field, thereby suppressing degradation of animage.

In this exemplary embodiment, mobile particles 34 will be described ashaving previously adjusted characteristics that contribute to themigration of mobile particles 34 in response to an electric field, suchas the average charge amount or electrostatic amount, so that mobileparticles 34 can move between display substrate 20 and rear substrate 22in response to an electric field formed between these substrates.

The adjustment of average charge amount of each mobile particle ofmobile particles 34 may be performed, specifically, by adjusting thetype or amount of charge control agent to be compounded in the resin asmentioned above, the type or amount of polymer chain to be bound to thesurface of the mobile particles, the type or amount of external additiveto be added or embedded into the surface of the mobile particles, thetype or amount of the surfactant, polymer chain or coupling agent to beapplied to the surface of the mobile particles, or the specific surfacearea of the mobile particles (such as the volume average particle sizeor the shape factor).

The production of mobile particles 34 may be performed by any knownmethod.

For example, as described in JP-A 7-325434, mobile particles 34 may beproduced by measuring a resin, a pigment and a charge controlling agentat a specific mixing ratio, melting the resin by heating and adding thepigment thereto and mixing and dispersing the same, cooling andpulverizing the same using a jet mill, a hammer mill or a turbo mill toprepare the mobile particles, and then dispersing the obtained mobileparticles in a dispersing medium.

Further, mobile particles 34 may be produced by preparing the mobileparticles including the charge control agent inside thereof by apolymerization method such as suspension-polymerization,emulsification-polymerization or dispersion-polymerization, or anaggregation method such as coacervation, melt dispersion oremulsion-aggregation, and then the obtained mobile particles in adispersing medium to prepare a dispersing medium including the mobileparticles.

Moreover, there is a method of using an appropriate device that performsdispersion, mixing and kneading of the resin, colorant, charge controlagent and/or dispersing medium at a temperature that is lower than thepoint of decomposition of the resin, colorant, charge control agentand/or dispersing medium, at which temperature the resin can plasticizeand the dispersing medium does not boil. Specifically, the mobileparticles can be obtained by mixing and heating to melt the pigment,resin and charge control agent in the dispersing medium using aplanetary mixer or a kneader, cooling the mixture while stirring usingthe temperature dependency of the solvent solubility of the resin, andthen allowing the mixture to coagulate/precipitate to form the mobileparticles.

Additionally, there is a method of producing the mobile particlesincluding placing the aforementioned raw materials in an appropriatecontainer equipped with particulate media for dispersing and kneading,such as an attritor or a heated vibrating mill such as a ball mill, andthen dispersing and kneading the content of the container at anappropriate temperature range, such as from 80 to 160° C. Preferredexamples of the material for the particulate media include steels suchas stainless steel or carbon steel, alumina, zirconia or silica. Whenproducing the mobile particles by this method, the raw materials thathave been previously made into a fluid state are further dispersed bythe particulate media in the container, and the resin including thecolorant is allowed to precipitate from the dispersing medium by coolingthe dispersing medium. The particulate media maintain the state ofmotion during the cooling and after the cooling, and reduce the size ofparticles by generating shearing force or impact strength.

The content of mobile particles 34 (weight %) with respect to the totalweight of the content of the cell is not particularly limited as long asthe desired color hue can be obtained. It is effective for displaymedium 12 to adjust the content of mobile particles 34 by adjusting thethickness of the cell (i.e., the distance between display substrate 20and rear substrate 22). Namely, in order to achieve the desired colorhue, the content of mobile particles 34 can be reduced (or increased) byincreasing (or reducing) the thickness of the cell. The content ofmobile particles 34 is typically from 0.01 to 50% by weight.

Reflective particles 36 are particles that are not charged and includeparticles having different optical reflection characteristics than thatof mobile particles 34, and function as a reflective member thatdisplays a different color from that of mobile particles 34. Further,reflective particle 36 function as a spacer which allows mobileparticles 34 to move through the space between display substrate 20 andrear substrate 22 without inhibiting the movement of mobile particles34. Namely, each particle of mobile particles 34 moves through gapsamong reflective particles 36 from the side of rear substrate 22 towardthe side of display substrate 20, or from the side of display substrate20 toward the side of rear substrate 22. The color of reflectiveparticles 36 may have, for example, a black color for the background,other than the white color. Further, reflective particles 36 may beparticles that are charged and move in response to an electric field.

When reflective particles 36 have a color other than white, reflectiveparticles may be resin particles including a colorant such as a pigmentor dye having a desired color. The pigment or dye may be those typicallyused in printing inks or color toners, such as those of RGB or YMCcolors.

Reflective particles 36 may be included between the substrates by aninkjet method or the like. Further, reflective particles 36 may befixed. In this case, for example, reflective particles 36 after beingincluded between the substrates are heated (and pressed if necessary) tomelt the surface of the particles, such that the gaps between theparticles are maintained. Reflective particles 36 may be filled betweenthe substrates, or may be suspended in a dispersing medium between thesubstrates.

The content of reflective particles 36 (% by weight) with respect to thetotal weight of the content of the cell is not particularly limited aslong as the desired color hue can be obtained. It is effective fordisplay medium 12 to adjust the content of reflective particles 36 byadjusting the thickness of the cell (i.e., the distance between displaysubstrate 20 and rear substrate 22). Namely, in order to achieve thedesired color hue, the content of reflective particles 36 can be reduced(or increased) by increasing (or reducing) the thickness of the cell.The content of reflective particles 36 is typically from 0.01 to 70% byweight.

The size of the cell in display medium 12 has a close relationship withthe definition of display medium 12, and display medium 12 that candisplay an image with a higher definition can be produced by reducingthe size of the cell. The cell typically has a length in a planedirection of display substrate 20 of from 10 μm to about 1 mm.

Display substrate 20 and rear substrate 22 can be fixed to each othervia spacers 24 using a combination of bolt and nut, a clamp, a clip, aflame for fixing the substrates, or the like. Alternatively, thesubstrates may be fixed to each other using an adhesive, or byperforming hot-melting, ultrasonic bonding, or the like.

Display medium 12 having the aforementioned structure is applicable tomedia that can record an image or re-writing an image, such as bulletinboards, circulars, electronic black boards, advertisements, billboards,flash signals, electronic paper, electronic newspapers, electronicbooks, and document sheets for use in both copiers and printers.

As mentioned above, display device according to this exemplaryembodiment includes display medium 12, voltage application unit 16 thatapplies a voltage to display medium 12, and control unit 18 (see FIG.1).

Voltage application unit 16 is electrically connected to front electrode40 and rear electrode 46. In the following, both of front electrode 40and rear electrode 46 are described as being electrically connected tovoltage application 16. However, it is also possible that one of theseelectrodes is grounded while the other is electrically connected tovoltage application 16.

Voltage application unit 16 is connected to control unit 18 such thatvoltage application unit 16 can give or receive signals.

Control unit 18 may be a microcomputer including a CPU (centralprocessing unit) that controls operation of the whole device, a RAM(random access memory) that temporarily records data of various kinds,and a ROM (read only memory) in which programs of various kinds, such ascontrol program for controlling the whole device, are recorded.

Voltage application unit 16 applies a voltage to front electrode 40 andrear electrode 46 in accordance with instructions from control unit 18.

In the following, the behavior of display device 10 will be described inaccordance with the operation of control unit 18.

Mobile particles 34 included in display medium 12 are described as blackand negatively charged. Dispersion medium 50 is described astransparent, and reflective particles 36 are described as white. Namely,in this exemplary embodiment, display medium 12 displays a black coloror a white color depending on the movement of mobile particles 34.

First, an initial operation signal is output to voltage application unit16. This signal indicates application of a voltage for a specified timeperiod, such that front electrode 40 serves as a negative electrode andrear electrode 46 serves as a positive electrode. When a voltage that isnegative and greater than a threshold voltage at which changes in theconcentration of particles stops is applied between the substrates,mobile particles 34 that are negatively charged move toward the side ofrear substrate 22, and reach rear substrate 22 (see FIG. 2A).

At this time, display medium 12 displays a white color of reflectiveparticles 36 at the side of display substrate 20.

The time T1 required for the above process may be recorded in advance ina memory such as a ROM (not shown) in control unit 18 as informationthat indicates the time for voltage application in the initialoperation, so that this information is read out upon execution of theoperation.

Subsequently, when a voltage having a polarity opposite to the voltagethat has been applied between the substrate is applied between theelectrodes such that front electrode 40 serves as a positive electrodeand rear electrode 46 serves as a negative electrode, mobile particles34 move toward display substrate 20 to reach display substrate 20. Atthis time, display medium 12 displays a black color of mobile particles34 (see FIG. 2B).

Second Exemplary Embodiment

In the following, a display device according to the second exemplaryembodiment will be described. FIG. 3 is a schematic view of a displaydevice according to the second exemplary embodiment of the invention,FIG. 4 is a diagram schematically showing the relationship between thevoltage and the degree of movement of particles (display density), andFIG. 5 is a schematic view showing the relationship between the mode ofvoltage applied between the substrates of the display medium and themode of movement of particles.

Display device 10 according to the second exemplary embodiment employstwo or more kinds of mobile particles 34, and these two or more kinds ofmobile particles 34 are charged to the same polarity. Display device 10according to the second exemplary embodiment employs particles having acolor other than white as mobile particles 34, while the display whiteparticles according to the invention are used as reflective particles36. It is also possible to employ the white particles for displayaccording to the invention as mobile particles 34 in the display device10 according to the first exemplary embodiment.

Display device 10 according to this exemplary embodiment includes, asshown in FIG. 3, display medium 12, voltage application unit 16 thatapplies a voltage to display medium 12, and control unit 18.

Since display device 10 according to this exemplary embodiment has asimilar structure to that of display device 10 according to the firstexemplary embodiment, the same components are provided with the sameindications and detailed explanations thereof are omitted.

Display medium 12 include display substrate 20, rear substrate 22 thatis positioned opposite to display substrate 20 with a gap therebetween,spacers 24 that retains these substrates to be positioned via apredetermined space and defines the space between the substrates intomultiple cells, mobile particles 34 included in each cell, andreflective particles 36 having an optical reflection characteristicsthat is different from that of mobile particle 34.

The surfaces of display substrate 20 and rear substrate 22 facing eachother are charge-treated as with the case of the first exemplaryembodiment, and surface layers 42 and 48 are provided on each of thesubstrate surfaces.

In this exemplary embodiment, two or more kinds of mobile particles 34having different colors are dispersed in dispersing medium 50.

In this exemplary embodiment, mobile particles 34 consist of yellowmobile particles 34Y having a yellow color, magenta mobile particles 34Mhaving a magenta color and cyan mobile particles 34C having a cyancolor. However, mobile particles 34 are not limited to these threecolors.

Mobile particles 34 move between the substrate by electrophoresis, andparticles of different colors in response to an electric field atdifferent absolute values of voltage. Namely, yellow mobile particles34Y, magenta mobile particles 34M and cyan mobile particles 34C moveupon application of voltage in a range that is different from eachother.

Mobile particles 34 including two or more kinds of particles that movein response to an electric field at different absolute values of voltagecan be obtained by preparing particle dispersions each containingparticles having different charge amounts, and then mixing theseparticles dispersions. The charge amount of particles can be adjustedby, for example, changing the amount of materials for mobile particles34 as described in the first exemplary embodiment, such as a chargecontrol agent or magnetic powder, or changing the type or concentrationof the resin that forms the particles.

As mentioned above, display medium 12 according to this embodimentincludes three kinds of mobile particles 34 dispersed in dispersingmedium 50, i.e., yellow mobile particles 34Y, magenta mobile particles34M and cyan mobile particles 34C. Mobile particles 34 of differentcolors move in response to an electric field upon application of avoltage at different absolute values.

In this exemplary embodiment, the absolute value of voltage at whichmagenta mobile particles 34M start to move is defined as |Vtm|, theabsolute value of voltage at which cyan mobile particles 34C start tomove is defined as |Vtc|, and the absolute value of voltage at whichyellow mobile particles 34Y start to move is defined as |Vty|,respectively. Further, the absolute value of maximum voltage at whichsubstantially all of magenta mobile particles 34M move is defined as|Vdm|, the absolute value of maximum voltage at which substantially allof cyan mobile particles 34C move is defined as |Vdc|, and the absolutevalue of maximum voltage at which substantially all of yellow mobileparticles 34Y move is defined as |Vdy|.

In the following, the relationship among the absolute values of Vtc,−Vtc, Vdc, −Vdc, Vtm, −Vtm, Vdm, −Vdm, Vty, −Vty, Vdy and −Vdy isdefined as |Vtc|<|Vdc|<|Vtm|<|Vdm|<|Vty|<|Vdy|.

Specifically, as shown in FIG. 4, for example, mobile particles 34 ofthree kinds are charged to the same polarity and are dispersed indispersing medium 50, and the range of absolute value of voltage atwhich cyan mobile particles 34C move |Vtc≦Vc≦Vdc| (absolute valuesbetween Vtc and Vdc), the range of absolute value of voltage at whichmagenta mobile particles 34M move |Vtm≦Vm≦Vdm| (absolute values betweenVtm and Vdm), and the range of absolute value of voltage at which yellowmobile particles 34Y move |Vty≦Vy≦Vdy| (absolute values between Vty andVdy) are set in this order such that these ranges do not overlap eachother.

Further, in order to move mobile particles 34 of each colorindependently from each other, the absolute value of maximum voltage atwhich substantially all of cyan mobile particles 34C move is set to beless than the range of absolute value of voltage at which magenta mobileparticles 34M move |Vtm≦Vm≦Vdm| (absolute values between Vtm and Vdm)and the range of absolute value of voltage at which yellow mobileparticles 34Y move |Vty≦Vy≦Vdy| (absolute values between Vty and Vdy).

Moreover, the absolute value of maximum voltage at which substantiallyall of magenta mobile particles 34M move is set to be less than therange of absolute value of voltage at which yellow mobile particles 34Ymove |Vty≦Vy≦Vdy| (absolute values between Vty and Vdy).

Therefore, in this exemplary embodiment, mobile particles 34 of eachcolor can be independently driven by setting the ranges of voltage atwhich mobile particles 34 of each color move do not overlap each other.

The range of voltage at which mobile particles 34 move is from a voltageat which particles start to move to a voltage at which the displaydensity stops to change (saturated) even when the amount of voltage andapplication time thereof are increased.

Further, the maximum voltage at which substantially all of mobileparticles 34 move is a voltage at which the display density stops tochange (saturated) even when the amount of voltage and application timethereof are increased since the start of movement.

The term “substantially all” is used since part of mobile particles 34may have different characteristics that do not contribute to the displaycharacteristics due to variation in characteristics of mobile particles34 of each color.

The “display density” refers to a density at which the density per unitof voltage stops to change (saturated), and is determined by measuringan optical density (OD) of color density at the display side, using areflective densiometer manufactured by X-Rite, Incorporated, whileapplying a voltage and changing the voltage between the substrates in adirection of increasing the density as measured (increasing ordecreasing the voltage for application) even when the amount of voltageand application time thereof are increased.

In display medium 12 according to this exemplary embodiment, when avoltage is applied between display substrate 20 and rear substrate 22and gradually increased from 0V to exceed +Vtc, display density startsto change due to the movement of cyan mobile particles 34C. When thevoltage is further increased to exceed +Vdc, the display density due tothe movement of cyan mobile particles 34C stops changing.

When the voltage is further increased to exceed +Vtm, display densitystarts to change due to the movement of magenta mobile particles 34M.When the voltage is further increased to +Vdm, the display density dueto the movement of magenta mobile particles 34M stops changing.

When the voltage is further increased to exceed +Vty, display densitystarts to change due to the movement of yellow mobile particles 34Y.When the voltage is further increased to +Vdy, display density due tothe movement of yellow mobile particles 34Y stops changing.

Conversely, when a voltage of minus polarity is applied between displaysubstrate 20 and rear substrate 22 and the absolute value of the voltageis gradually increased from 0V to exceed −Vtc, display density starts tochange due to the movement of cyan mobile particle 34C. When theabsolute value of voltage is further increased to −Vdc, the displaydensity due to the movement of cyan mobile particles 34C stops changing.

When the absolute value of voltage is further increased to exceed −Vtm,display density starts to change due to the movement of magenta mobileparticles 34M. When the absolute value of voltage is further increasedto −Vdm, the display density due to the movement of magenta mobileparticles 34M stops changing.

When the absolute value of voltage is further increased to exceed −Vty,display density starts to change due to the movement of yellow mobileparticles 34Y. When the absolute value of voltage is further increasedto −Vdy, the display density due to the movement of yellow mobileparticles 34Y stops changing.

Accordingly, in this exemplary embodiment as shown in FIG. 4, when avoltage in a range of from −Vtc to +Vtc (|Vtc| or less) is appliedbetween display substrate 20 and rear substrate 22, movement of cyanmobile particles 34C, magenta mobile particles 34M and yellow mobileparticles 34Y does not occur at such a level that the display density indisplay medium 12 changes. When a voltage having an absolute value thatis more than +Vtc or −Vtc is applied between the substrates, cyan mobileparticles 34C (in cyan mobile particles 34C, magenta mobile particles34M and yellow mobile particles 34Y) start to move at such a level thatcauses changes in display density in display medium 12, and when avoltage having an absolute value that is more than +Vdc or −Vdc isapplied between the substrates, the display density per unit voltagestops changing.

Further, when a voltage in a range of from −Vtm to +Vtm (|Vtm| or less)is applied between display substrate 20 and rear substrate 22, movementof magenta mobile particles 34M and yellow mobile particles 34Y does notoccur at such a level that the display density in display medium 12changes. When a voltage having an absolute value that is more than +Vtmor −Vtm is applied between the substrates, magenta mobile particles 34M(in magenta mobile particles 34M and yellow mobile particles 34Y) startto move at such a level that causes changes in display density indisplay medium 12, and when a voltage having an absolute value of |Vdm|or more is applied between the substrates, the display density stopschanging.

Further, when a voltage in a range of from −Vty to +Vty (|Vty| or less)is applied between display substrate 20 and rear substrate 22, movementof yellow mobile particles 34Y does not occur at such a level that thedisplay density in display medium 12 changes. When a voltage having anabsolute value that is more than +Vty or −Vty is applied between thesubstrates, yellow mobile particles 34Y start to move at such a levelthat causes changes in display density in display medium 12, and when avoltage having an absolute value of |Vdy| or more is applied between thesubstrates, the display density stops changing.

Subsequently, the mechanism of how the particles move when an image isdisplayed in display medium 12 will be described with reference to FIG.5.

For example, display medium 12 includes yellow mobile particles 34Y,magenta mobile particles 34M and cyan mobile particles 34C as explainedwith reference to FIG. 4 as mobile particles 34 of plural kinds.

In the following, the voltage to be applied between the substrates thatis more than an absolute value at which yellow mobile particles 34Ystart moving but not more than a maximum voltage at which substantiallyall of yellow mobile particles 34Y move is referred to as “voltage L”,the voltage to be applied between the substrates that is more than anabsolute value at which magenta mobile particles 34M start moving butnot more than a maximum voltage at which substantially all of magentamobile particles 34M move is referred to as “voltage M”, and the voltageto be applied between the substrates that is more than an absolute valueat which cyan mobile particles 34C start moving but not more than amaximum voltage at which substantially all of cyan mobile particles 34Cmove is referred to as “voltage S”.

When the voltage applied between the substrates is higher at the side ofdisplay substrate 20 than the side of rear substrate 22 is appliedbetween the substrates, the above voltages are referred to as “+voltageL”, “+voltage M” and “+voltage S”, respectively. When the voltageapplied between the substrates is higher at the side of rear substrate22 than the side of display substrate 20, the above voltages arereferred to as “−voltage L”, “−voltage M” and “−voltage S”,respectively.

As shown in FIG. 5, for example, all of magenta mobile particles 34M,cyan mobile particles 34C and yellow mobile particles 34Y are positionedat the side of rear substrate 22 to display a white color at the initialstate (see (A)). When +voltage L is applied between display substrate 20and rear substrate 22 at this initial state, all of magenta mobileparticles 34M, cyan mobile particles 34C and yellow mobile particles 34Ymove to the side of display substrate 20. These particles remain at theside of display substrate 20 even when the voltage application isstopped at this state, thereby exhibiting a black color formed bysubtractive color mixing of magenta, cyan and yellow (see (B)).

Subsequently, when -voltage M is applied between display substrate 20and rear substrate 22 in the state of (B), magenta mobile particles 34Mand cyan mobile particles 34C move to the side of rear substrate 22. Asa result, only yellow mobile particles 34Y remain at the side of displaysubstrate 20, thereby exhibiting a yellow color (see (C)).

Further, when +voltage S is applied between display substrate 20 andrear substrate 22 in the state of (C), cyan mobile particles 34C move tothe side of display substrate 22. As a result, yellow mobile particles34Y and cyan mobile particles 34C are positioned at the side of displaysubstrate 20, thereby exhibiting a green color formed by subtractivecolor mixing of cyan and yellow (see (D)).

When −voltage S is applied between display substrate 20 and rearsubstrate 22 in the state of (B), cyan mobile particles 34C move to theside of rear substrate 20. As a result, yellow mobile particles 34Y andmagenta mobile particles 34M are positioned at the side of displaysubstrate 20, thereby exhibiting a red color formed by subtractive colormixing of yellow and magenta (see (I)).

When +voltage M is applied between display substrate 20 and rearsubstrate 22 in the state of (A), magenta mobile particles 34M and cyanmobile particles 34C move to the side of display substrate 20. As aresult, magenta mobile particles 34M and cyan mobile particles 34C arepositioned at the side of display substrate 20, thereby exhibiting ablue color formed by subtractive color mixing of magenta and cyan (see(E)).

When −voltage S is applied between display substrate 20 and rearsubstrate 22 in the state of (E), cyan mobile particles 34C move to theside of rear substrate 22. As a result, only magenta mobile particles34M are positioned at the side of display substrate 20, therebyexhibiting a magenta color (see (F)).

When −voltage L is applied between display substrate 20 and rearsubstrate 22 in the state of (F), magenta mobile particles 34M move tothe side of rear substrate 22. As a result, no mobile particles arepositioned at the side of display substrate 20, thereby exhibiting awhite color of reflective particles 36 (see (G)).

When +voltage S is applied between display substrate 20 and rearsubstrate 22 in the initial state of (A), cyan mobile particles 34C moveto the side of display substrate 20. As a result, cyan mobile particles34C are positioned at the side of display substrate 20, therebyexhibiting a cyan color (see (H)).

When −voltage L is applied between display substrate 20 and rearsubstrate 22 in the state of (I), all of mobile particles 34 move to theside of rear substrate 22. As a result, no mobile particles arepositioned at the side of display substrate 20, thereby exhibiting awhite color of reflective particles 36 (see (G)).

Similarly, when −voltage L is applied between display substrate 20 andrear substrate 22 in the state of (D), all of mobile particles 34 moveto the side of rear substrate 22. As a result, no mobile particles arepositioned at the side of display substrate 20, thereby exhibiting awhite color of reflective particles 36 (see (G)).

In this exemplary embodiment, a voltage corresponding to each kind ofmobile particles 34 is applied between the substrates. Therefore,desired particles can be selectively moved in response to an electricfield formed by the voltage, migration of particles of other colors indispersing medium 50 can be suppressed, thereby suppressing mixing of anundesired color. As a result, a color can be displayed while suppressingimage degradation of display medium 12.

A vivid color can be displayed as long as mobile particle 34 ofdifferent colors move upon application of a voltage having differentabsolute values, even if the ranges of the voltage overlap each other.However, when the ranges of voltage do not overlap each other, mixing ofcolors can be more suppressed and a more vivid color display can berealized.

Further, by dispersing mobile particles 34 of cyan, magenta and yellowin dispersing medium 50, colors of cyan, magenta, yellow, blue, red,green and black can be displayed and, for example, a white color can bedisplayed by using white reflective particle 36, thereby enablingdisplay of a specific color.

As mentioned above, in display device 10 according to this exemplaryembodiment, display can be performed by mobile particles 34 that havearrived at display substrate 20 or rear substrate 22.

EXAMPLES

In the following, the invention will be described in further detailswith reference to the Examples, but the invention is not limitedthereto.

Example 1

—Preparation of White Mother Particles A—

300 g of polymethylphenylsilane (trade name: SI-10-10, manufactured byOsaka Gas Chemicals Co., Ltd., refractive index: 1.7, specific gravity:1.0, structure: (I-IA) where n=60) are pulverized using a supersonic jetmill (trade name: IDS-2, manufactured by Nippon Pneumatic Mfg. Co.,Ltd.) and particles having a volume average particle size of 1.5 μm areobtained. These particles are classified, and particles of a polysilanecompound having a volume average particle size of 1.0 μm are obtained aswhite mother particles A.

—Preparation of Reactive Silicone Polymer A—

30 parts by weight of a silicone monomer as a silicone chain component(SILAPLANE FM-0721, trade name, manufactured by Chisso Corporation,volume average molecular weight: 5,000), 5 parts by weight ofdiethylaminoethyl methacrylate (DEAEMA) as a monomer having a charginggroup (a component having a charging group) and 65 parts by weight ofhydroxymethacrylate as a monomer having no charging group (othercopolymerization component) are mixed in 300 parts by weight ofisopropyl alcohol (IPA), and 1 part by weight of AIBN(2,2′-azobisisobutyl nitrile) is dissolved therein. The mixture isallowed to polymerize under a nitrogen atmosphere at 60° C. for 24hours. The obtained product is purified using hexane as are-precipitation solvent and then dried, thereby obtaining a siliconepolymer A.

—Preparation of White Particles 1 (Dispersion 1)—

1 g of white mother particles A and 0.4 g of the silicone polymer A aredissolved and dispersed in 10 g of IPA, and mixed by stirring for 6hours. This solution is emulsified while gradually adding 20 g of 2CSsilicone oil (trade name: KF96, manufactured by Shin-Etsu Chemical Co.,Ltd.). Then, the solution is intermittently stirred with an ultrasonichomogenizer for 1 hour while cooling the solution at 30° C. Thereafter,the solution is heated to 50° C. and dried with reduced pressure toevaporate the IPA. White particle dispersion 1 having a volume averageparticle diameter of 1.0 μm is thus obtained.

The charge polarity of particles in the dispersion is determined byincluding the dispersion between a pair of electrode substrates andapplying a direct current thereto. The direction in which the particlesmove is evaluated. As a result, the particles are positively charged.

Example 2

—White Mother Particles B—

A mixture of 10 g of polydiphenylsilane (trade name: SI-30-10,manufactured by Osaka Gas Chemicals Co., Ltd., refractive index: 1.74,specific gravity: 1.0, structure: (I-2A)) and 40 g of IPA is pulverizedfor 60 hours using a ball mil (trade name: IDS-2, manufactured by NipponPneumatic Mfg. Co., Ltd.) with 20 g of zirconia beads having a diameterof 2 mm, and particles of a polysilane compound having a volume averageparticles size of 0.2 μm are obtained as white mother particles B.

—Preparation of White Particles 2 (Dispersion 2)—

1 g of white mother particles B and 0.5 g of silicone polymer A aredissolved and dispersed in 10 g of IPA, and mixed by stirring for 6hours. This solution is emulsified while gradually adding 20 g of 2CSsilicone oil (trade name: KF96, manufactured by Shin-Etsu Chemical Co.,Ltd.). Then, the solution is intermittently stirred with an ultrasonichomogenizer for 1 hour while cooling the solution at 30° C. Thereafter,the solution is heated to 50° C. and dried with reduced pressure toevaporate the IPA. White particle dispersion 2 having a volume averageparticle diameter of 0.2 μm is thus obtained.

The charge polarity of the particles in the dispersion is determined byincluding the dispersion between a pair of electrode substrates andapplying a direct current thereto. The direction in which the particlesmove is evaluated. As a result, the particles are positively charged.

Comparative Example 1

—Preparation of Dispersion A1—

Dispersion A1 is prepared by mixing the following components andpulverizing the same for 20 hours using a ball mill with zirconia ballshaving a diameter of 10 mm.

<Composition>

Cyclohexyl methacrylate 61 parts by weight Divinyl methoxy silane  1part by weight Titanium oxide 1 (white pigment) 35 parts by weight(Primary particle size: 0.3 μm, trade name: TIPAQUE CR63, manufacturedby Ishihara Sangyo Kaisha, Ltd.) Hollow particles  3 parts by weight(Primary particle size: 0.3 μm, trade name: SX866(A), manufactured byJSR Corporation) Charge control agent  1 part by weight (Trade name:SBT-5-0016, manufactured by Orient Chemical Industries Ltd.)

—Preparation of Calcium Carbonate Dispersion B1—

Calcium carbonate dispersion B1 is prepared by mixing the followingcomponents and finely pulverizing the same using a ball mill in asimilar manner to the above.

<Composition>

Calcium carbonate 40 parts by weight Water 60 parts by weight

—Preparation of Mixed Solution C1—

Mixed solution C1 is prepared by mixing the following components anddegassing the same using an ultrasonic machine for 10 minutes, and thenstirring the same using an emulsifier.

<Composition>

Calcium carbonate dispersion B1 8.5 g 20% salt water  50 g

—Preparation of Comparative White Particles—

Subsequently, 35 g of dispersion A1, 1 g of ethylene glycoldimethacrylate and 0.35 g of polymerization initiator (AIBN) arethoroughly mixed and degassed for 2 minutes using an ultrasonic machine.The resultant is added to mixed solution C1 as prepared above, and themixture is emulsified using an emulsifier. The obtained emulsifiedsolution is placed in a bottle and sealed with a silicone cap. Thecontent of the bottle is thoroughly degassed using an injection needle,and the bottle is filled with a nitrogen gas. The content of the bottleis allowed to react at 65° C. for 15 hours at this sate to formparticles. The obtained particle powder is dispersed in ion exchangewater. Then, the calcium carbonate is decomposed using hydrochloric acidwater and filtered. Thereafter, the particle powder is washed with asufficient amount of distilled water, and unclassified white particlesare obtained. The white particles are passed through nylon sieves eachhaving an opening of 10 μm and 15 μm to regulate the particle size. Thewhite particles are dried, and comparative white particles having avolume average particle size of 13 μm and a specific gravity of 1.7 arethus obtained. 2.0 g of comparative white particles are dispersed in 20g of 2CS silicone oil (trade name: KF96, manufactured by Shin-EtsuChemical Co., Ltd.), and the comparative white particle dispersion isobtained.

The charge polarity of the particles in the dispersion is determined byincluding the dispersion between a pair of electrode substrates andapplying a direct current thereto. The direction in which the particlesmove is evaluated. As a result, the particles are negatively charged.

(Evaluation)

Using the white particle dispersions as prepared in the above Examplesand Comparative Example, sample devices are prepared by the followingmethod.

Specifically, the white particles dispersion, cyan particle dispersion(below) and a silicone oil (2CS silicone oil, trade name: KF96,manufactured by Shin-Etsu Chemical Co., Ltd.) are mixed in accordancewith Table. 1, and the mixture is included in a cell formed between apair of glass substrates on which an indium tin oxide (ITO) electrode isformed, with a spacer of 50 μm.

—Cyan Particle Dispersion—

65 parts by weight of hydroxyethyl methacrylate, 30 parts by weight ofSILAPLANE FM-0721 (trade name, manufactured by Chisso Corporation,volume average molecular weight: 5,000) and 5 parts by weight ofmethacrylic acid are mixed in 100 parts by weight of isopropyl alcohol,and 0.2 parts by weight of AIBN are dissolved therein as apolymerization initiator. Then, the mixture is allowed to polymerizeunder a nitrogen atmosphere at 70° C. for 6 hours. The obtained productis purified using hexane as a re-precipitation solvent and dried,thereby obtaining a polymer.

Subsequently, 0.5 g of the above polymer is dissolved in 9 g ofisopropyl alcohol. Then, 0.5 g of a cyan pigment (CYANINE BLUE 4973,manufactured by Sanyo Color Works, Ltd.) are added thereto and dispersedfor 48 hours using zirconia balls having a diameter of 0.5 mm, therebyobtaining a pigment-containing polymer solution.

To 3 g of this pigment-containing polymer solution, 12 g of 2CS siliconeoil (trade name: KF96, manufactured by Shin-Etsu Chemical Co., Ltd.) isgradually dropped and then emulsified while applying ultrasonic waves.Thereafter, the solution is heated to 60° C. and dried with reducedpressure to evaporate the IPA, thereby obtaining particles for displayincluding a polymer and a pigment. The particles for display are allowedto settle using a centrifuge separator and a supernatant liquid isremoved. 5 of the above silicone oil are further added thereto andultrasonic waves are applied, washed and particles are allowed to settleusing a centrifuge separator, and the supernatant liquid is removed.Then, 5 of the above silicone oil are further added thereto, and a cyanparticle dispersion is thus obtained. The volume average particle sizeof the cyan particle is 0.2 μm.

The charge polarity of the particles in the dispersion is determined byincluding the dispersion between a pair of electrode substrates andapplying a direct current thereto. The direction in which the particlesmove is evaluated. As a result, the particles are negatively charged.

—Evaluation Method—

A direct current having a voltage of 10 V is applied to both of theelectrodes of the sample device, and then the polarity is reversed tomove the particles for display. When a positive voltage is applied tothe display side electrode, the cyan particles move to the display sideglass substrate to display a cyan color. On the other hand, when anegative voltage is applied to the display side electrode, the cyanparticles move to the rear side glass substrate to display a whitecolor. When the white particles are positively charged, the white coloris displayed by the movement of the white particles toward the displayside glass substrate. When the white particles are not charged, thewhite color is displayed only by the movement of the cyan particles.

After performing the display of cyan color and white color for 100times, the white color is displayed by the sample device. At this state,the sample device is left to stand for 5 hours, and the degree ofwhiteness of the display is evaluated in accordance with the followingcriteria. The results are shown in Table 1. Further, the degree ofsedimentation of the white particles is also evaluated in accordancewith the following criteria. The results are shown in Table 1.

—Evaluation of Whiteness—

The degree of whiteness of display is evaluated by measuring a whitereflection density using a color reflection densitometer (trade name:X-Rite 404, manufactured by X-Rite Corporation), and then calculatingthe whiteness by the following formula. The difference between thewhiteness as measured at the initial stage and the whiteness as measuredafter repeating the display process for 100 times is evaluated accordingto the following criteria.

Whiteness (white reflectanceratio)=10^(−(white reflection density))×100%

A: 0 to 10%

B: 11 to 15%

C: 16 to 20%

D: 21% or more

—Degree of Sedimentation of White Particles—

The degree of sedimentation of the white particles are evaluated byplacing the dispersion in a colorless, transparent glass sample tubehaving a volume of 8 ml, and the sample tube is left to stand for 3hours, 10 hours, 24 hours and 48 hours, respectively. The state ofsupernatant liquid is observed in accordance with the followingcriteria.

A: Supernatant liquid is slightly transparent after 48 hours.

B: Supernatant liquid is slightly transparent after 24 hours.

C: Supernatant liquid is slightly transparent after 10 hours.

D: Most particles form a sedimentation and supernatant liquid istransparent after 3 hours.

TABLE 1 White Cyan particle particle Silicone dispersion dispersion oilWhiteness Sedimentation Notes Example 1 20 parts 1 part by 6 parts by BB (white particle by weight weight weight dispersion 1) Example 2 20parts 1 part by 6 parts by A A (white particle by weight weight weightdispersion 2) Comparative 30 parts 1 part by 6 parts by D D * Example 1by weight weight weight (comparative white particle dispersion) * Abluish white color is displayed due to sedimentation of the whiteparticles.

As shown by the above result, Examples 1 and 2 display a white colorwith a high degree of whiteness while suppressing sedimentation of thewhite particles, as compared with Comparative Example 1.

All publications, patent applications, and technical standards mentionedin this specification are herein incorporated by reference to the sameextent as if each individual publication, patent application, ortechnical standard was specifically and individually indicated to beincorporated by reference.

1. White particles for display comprising at least one of a chain orcyclic polysilane compound having a polysilane structure represented bythe following Formula (I) or a halogen-substituted compound thereof:

wherein in Formula (I), A represents a phenyl group, B represents analkyl group or a phenyl group, and n represents an integer of from 5 to1000.
 2. The white particles for display according to claim 1, whereinthe polysilane compound comprises a chain polysilane compound having astructure represented by the following Formula (I-1A):

wherein in Formula (I-1A), n represents an integer of from 5 to
 1000. 3.The white particles for display according to claim 1, wherein thepolysilane compound comprises a cyclic polysilane compound having astructure represented by the following Formula (I-2A):


4. A particle dispersion for display comprising white particles fordisplay and a dispersing medium for dispersing the white particles fordisplay, the white particles for display comprising at least one of achain or cyclic polysilane compound having a polysilane structurerepresented by the following Formula (I) or a halogen-substitutedcompound thereof:

wherein in Formula (I), A represents a phenyl group, B represents analkyl group or a phenyl group, and n represents an integer of from 5 to1000.
 5. The particle dispersion for display according to claim 4,wherein the polysilane compound comprises a chain polysilane compoundhaving a structure represented by the following Formula (I-1A):

wherein in Formula (I-1A), n represents an integer of from 5 to
 1000. 6.The particle dispersion for display according to claim 4, wherein thepolysilane compound comprises a cyclic polysilane compound having astructure represented by the following Formula (I-2A):


7. A display medium comprising: a pair of substrates facing each otherwith a space therebetween, at least one of the substrates beingtransparent; color particles that are located between the substrates andmove between the substrates in response to an electric field formedbetween the substrates; white particles for display that are locatedbetween the substrates; and a dispersing medium that is located betweenthe substrates and disperses the color particles and the white particlesfor display, the white particles for display comprising at least one ofa chain or cyclic polysilane compound having a polysilane structurerepresented by the following Formula (I) or a halogen-substitutedcompound thereof:

wherein in Formula (I), A represents a phenyl group, B represents analkyl group or a phenyl group, and n represents an integer of from 5 to1000.
 8. The display medium according to claim 7, wherein the polysilanecompound comprises a chain polysilane compound having a structurerepresented by the following Formula (I-1A):

wherein in Formula (I-1A), n represents an integer of from 5 to
 1000. 9.The display medium according to claim 7, wherein the polysilane compoundcomprises a cyclic polysilane compound having a structure represented bythe following Formula (I-2A):


10. A display medium comprising: a pair of electrodes; white particlesfor display that are located between the electrodes and move between theelectrodes in response to an electric field formed between theelectrodes; and a dispersing medium that is located between theelectrodes and disperses the white particles for display, the whiteparticles for display comprising at least one of a chain or cyclicpolysilane compound having a polysilane structure represented by thefollowing Formula (I) or a halogen-substituted compound thereof:

wherein in Formula (I), A represents a phenyl group, B represents analkyl group or a phenyl group, and n represents an integer of from 5 to1000.
 11. The display medium according to claim 10, wherein thepolysilane compound comprises a chain polysilane compound having astructure represented by the following Formula (I-1A):

wherein in Formula (I-1A), n represents an integer of from 5 to 1000.12. The display medium according to claim 10, wherein the polysilanecompound comprises a cyclic polysilane compound having a structurerepresented by the following Formula (I-2A):


13. A display device comprising the display medium according to claim 7and an electric field formation unit that forms an electric fieldbetween the pair of substrates.
 14. A display device comprising thedisplay medium according to claim 10 and an electric field formationunit that forms an electric field between the pair of electrodes.